Post work is a lot of different things(Sound, editing, looping). So yeah, they don't have a gun on their heads to finish but I don't think they're going to wait til the last minute on effects just because they have time to twiddle their thumbs.

Every major effects driven film is rendering up until the last possible minute. That is standard practice in the industry these days. If they have five extra months, you'd better believe they're going to use every last second they have. That's the antithesis of twiddling their thumbs.